1. Field of the Invention
This invention relates to a process for producing a peroxide-vulcanizable, fluorine-containing elastomer, and more particularly to a process for producing a fluorine-containing elastomer having those halogen atoms in the molecule which can act as cross-linking points for peroxide vulcanization.
2. Description of the Prior Art
Generally, vulcanized fluorine-containing elastomers have distinguished heat resistance, solvent resistance, weathering and ozone resistances, creep resistance, etc., and are commercially widely used as a sealing material for oil seal, packing material, gasket, O ring, etc., or as a diaphragm material, a hose lining material, a coating material, an adhesive, etc.
One of the conventional processes for obtaining such a vulcanized, fluorine-containing elastomer is based on a peroxide vulcanization method using an organic peroxide as a vulcanizing agent, where fluorine-containing elastomers having iodine atoms or bromine atoms as cross-linking points are used. Some of the examples will be given below, and they have the following problems.
Japanese Patent Application Kokai (Laid-open) No. 53-125,491 discloses a process using an iodine compound represented by the general formula RfIx, wherein Rf represents a fluorohydrocarbon group or a chlorofluorohydrocarbon group, but this iodine compound is expensive and highly toxic and readily liberates the iodine atoms bonded to the fluorine-containing elastomer, when exposed to light, as disclosed in the following Japanese Patent Application Kokai (Laid-open) No. 60-221,409.
Japanese Patent Application Kokai (Laid-open) No. 60-221,409, discloses a process using an iodine compound represented by the general formula RI.sub.1-2, wherein R represents a hydrocarbon group having 1 to 3 carbon atoms, and this iodine compound is cheap and less toxic than the said compound represented by RfIx, but is poor in the vulcanization speed, and the heat resistance and compression set of the vulcanized products.
Japanese Patent Application Kokai (Laid-open) No. 59-20,310 discloses a process using a bromine compound represented by the general formula RBrx, wherein R represents a saturated aliphatic hydrocarbon group. Inventors of this prior art invention are partly common to those of the prior art invention disclosed in the said Japanese Patent Application Kokai (Laid-open) No. 53-125,491 and state that the fluorine-containing elastomers having bromine atoms are superior in the properties than the said fluorine-containing elastomers having iodine atoms. Indeed, the fluorine-containing elastomers having bromine have a better light stability, but are poor in the vulcanization speed and the heat resistance and compression set of the vulcanized products.
Japanese Patent Publication No. 54-1,585 discloses a process using a brominated olefin compound such as bromotrifluoroethylene, 4-bromo-3,3,4,4-tetrafluorobutene-1, etc., but the thus obtained fluorine-containing elastomers are liable to undergo gelation and are not only poor in the processability (flow characteristics), but also unsatisfactory in the elongation and compression set of the vulcanized products.
Japanese Patent Application Kokai (Laid-open) No. 60-195,113 discloses a process for copolymerizing not more than about 5% by mole, preferably 0.1 to 1.5% by mole, of a vinyl ether monomer acting as a curing site, represented by the general formula: EQU ROCX.dbd.CYZ
wherein one or two of X, Y and Z is selected from bromine and iodine atoms, the remainder being hydrogen, fluorine or chlorine atom, R is a linear or cyclic alkyl group, or alkenyl group or allyl group, but this prior art has the same disadvantages as in the art of the said Japanese Patent Publication No. 54-1,585.
Japanese Patent Application Kokai (Laid-open) No. 62-36,407 discloses a process for copolymerizing brominated fluorovinylether represented by the general formula: EQU CF.sub.2 Br(Rf)nOCF.dbd.CF.sub.2
wherein Rf is a fluorinated alkylene group and n is 0 or 1, in the presence of a chain transfer agent represented by the general formula R(CF.sub.2 BR)m, wherein R is bromine, a containing perfluoroalkyl group or a bromine-containing alkylene group, and m is 1 or 2, where only a vulcanized product with a poor permanent set is obtained due to the cross-linking only by bromine.
As a result of extensive studies on a process for producing a peroxide-vulcanizable, fluorine-containing elastomer that can give a vulcanized product with distinguished processability, vulcanization characteristics and vulcanization physical properties, free from the problems encountered in the prior arts, the present inventors have previously found that the problems can be effectively solved by polymerizing a fluorine-containing olefin in the presence of an iodine and bromine-containing compound, thereby introducing and iodines and bromines into the fluorine-containing elastomer molecule as cross-linking points simultaneously (U.S. Pat. No. 4,748,223). Though the desired object was attained by the previous finding, the peroxide-vulcanized product of the fluorine-containing elastomer had a compression set of more than 30%, and thus a further improvement of the permanent set has been still desired.
As a result of further investigation of a process capable of solving such a new problem, the present inventors found that the problem was effectively dissolved by conducting the polymerization reaction in the simultaneous presence of the iodine and bromine-containing compound and a small amount of 1,1-difluoro-2-bromoethylene, and the resulting copolymerization of the 1,1-difluoro-2-bromoethylene (U.S. Pat. No. 4,943,622).
Thus, the present inventors succeeded in considerable improvement of the compression set of the fluorine-containing elastomers by using the iodine and bromine-containing compound that deemed to act as a chain transfer agent together with 1,1-difluoro-2-bromoethylene.